Azobenzene (Az) groups were planted on the pore wall of mesoporous silica MCM-41 (M41) by silylation of triethoxy[4-phenylazo(phenyl)]silane. The optimal surface density of Az groups was 0.9 group nm(-2), and too much loading of Az induced the lowering of the efficiency of the trans-cis isomerization due to the congestion of the groups. The reversible change in the pore diameters upon UV-vis irradiation could not be confirmed by N-2 adsorption at 77 K but was revealed to be ca. 1.0 nm by the shift of the UV-vis absorption band of p-N,N-dimethylaminobenzylidenemalononitrile introduced into the Az-modified pores.